Many variations on mixing valves for showers and baths have been developed and marketed. These include thermostatically controlled valves and pressure balanced valves. Typically a sensing-controlling element, such as a thermostatic expansion device or a pressure sensing-and-balancing piston senses a change in flow conditions and automatically opens or closes orifices to compensate for the change. To be effective the sensing-controlling element must "see" and act upon the incoming water flow upstream of any temperature setting element or mechanism.
Mixing valves of the type comprising a water pressure-sensing- and-balancing piston are exemplified by those disclosed in U.S. Pat. Nos. 2,308,127; 3,099,996; and 3,448,755. In such valves the hot and cold water sources are applied to opposite ends of the piston, and as pressure variations take place the piston is caused to move under the action of the pressure difference that occurs. The water flows through orifices controlled by the piston to another orifice pair that are set to proportion the hot/cold flow mix.
Since the sensing element, i.e., the water-pressure equalizing piston, must "see" and act upon the source water supply pressure, it must be connected directly across the hot and cold water supplies. Accordingly, if a leak occurs, hot water could be introduced into the cold water supply and vice-versa. To prevent that occurrence it is necessary to have a positive shut-off located up-stream of the sensing element. In mixing valves of the type described in the above-identified U.S. Patents, which type is more recently exemplified by the Temptrol.RTM. valves produced by Symmons Industries, Inc. of Braintree, Mass., this is accomplished by including two elastomeric "seats" that positively shut off the hot/cold supplies up-stream of the pressure balancing element. These seats are incorporated directly on the temperature setting, hot/cold ratioing element (the spindle assembly), obviating the need for separate check-valves as in many competing devices.
Also prior to the present invention it was recognized that use of ceramic members as valving elements in hot and cold water mixing valves offers several advantages, and a number of different water flow control products using ceramic components have been marketed. For example, kitchen and lavatory faucets using ceramic valving components have been developed and/or marketed by a number of companies, including Masco Corporation. See also U.S. Pat. No. 3,788,354, issued Jan. 29,1974 to Paul C. Symmons for Single Handle Water Mixing Valve. The use of ceramic valving elements in shower valves of the type having water pressure balancing piston elements is disclosed by U.S. Pat. No. 3,921,659 issued Nov. 25, 1975 to Charles J. Rudewick III for Modular Balanced Pressure Mixing Valve With Ceramic Valve Elements. Additionally Zurn Industries of Dallas, Tex. has marketed a shower valve using ceramic elements. However, to Applicant's knowledge, in the Zurn Company shower valve the ceramic elements are arranged so as to provide temperature-ratioing and shut-off control downstream of the pressure balancing element and, therefore, auxiliary check valves must be incorporated between the shower valve and the hot and cold water supplies to prevent backflow into the hot and cold water supply lines.
Non-scald shower and bath mixing valves using water pressure sensing-and-equalizing piston elements, notably valves having operating modes similar to those disclosed in said U.S. Pat. Nos. 2,308,127; 3,099,996; and 3,448,755, have achieved extensive commercial success because they have effectively eliminated the danger of accidental scalding resulting from a rapid change in water temperature as a consequence of a variation in water pressure, and also because their elastomeric seats positively shut off the hot and cold water supplies up-stream of the water pressure sensing and balancing means.
Nevertheless it has been recognized that there is a need to improve upon existing designs of non-scald mixing valves in a way that lowers manufacturing costs without any loss of non-scald protection and without requiring the use of check valves in the hot and cold water supply lines.